Circumsporozoite (CS) proteins have been shown to act as the protective antigens capable of inducing sporozoite - specific immunity in several species of malaria. The gene coding for this antigen in Plasmodium falciparum has been isolated, cloned, and incorporated into E. coli. The recombinant protein induces antibodies in mice which react with sporozoites and blocks liver invasion in vitro. T cells are important for the development of protective immunity to malarial sporozoites. The repeated epitope, NANP, of the P. falciparum circumsporozoite (CS) protein is recognized by T cells from mice which carry the I-AB gene. Evidence suggests that the human immune response to this epitope may be very restricted. This has serious implications for vaccine development.